Pipes, in particular pressure pipes, are used in various applications like high temperature resistant pipe applications.
However, polyethylene has a limited pressure resistance at elevated temperature. Especially, it is difficult to combine good pressure resistance at higher temperatures with a high flexibility of the piping materials.
For improving pipe performance at elevated temperature density of the polyethylene pipe resin can be increased. However, increasing density as the additional effect of increasing the flexural and the tensile modulus of the material, which makes the material stiffer and less flexible. Furthermore, increasing density renders the polyethylene material more brittle. For many applications, however, such as e.g. floor heating, flexible pipes are preferred due to easier installation of flexible pipes.
Alternatively, pipe performance at elevated temperature can be improved by decreasing the melt flow rate MFR5 of the polyethylene resin. However, a lower MFR5 has a negative effect on the processability of the material which limits the maximum line speed and also might lead to polymer degradation during pipe extrusion.
In several applications, such as e.g. heat exchangers, wide temperature range from −40° C. to 70° C. is desired. The need for the wide service temperature range limits the usability of polypropylene random copolymer pipe grades in these applications. Polypropylene random copolymer pipe grades have a relatively good hydrostatic pressure resistance at elevated temperatures, but due to inherent poor mechanical performance of polypropylene at temperatures below 0° C. they cannot be used in applications in which low temperature performance is required.
The main challenge in the development of polyethylene grade for the above described applications has been to find the optimal balance between flexibility, processability and good pipe surface quality while still meeting the requirements of DIN 16833 and a good mechanical performance at low temperatures.
WO 03/033586 describes polyethylene pipes made of ethylene/1-butene copolymer resins for hot fluid applications. Although meeting the DIN 16833 requirements the mechanical performance of these materials over a broad temperature range needs to be improved.
EP 1 927 627 describes polyethylene pipes with improved high temperature resistance. The polyethylene resins are polymerized in the presence of a metallocene catalyst which results in good mechanical performance. However, due to the narrow molecular weight distribution of the resins the processability and pipe surface quality is low.
There is a need for polyethylene resins which overcome the above mentioned drawbacks and show the optimal balance between flexibility, processability and good pipe surface quality while still meeting the requirements of DIN 16833 and a good mechanical performance over a broad temperature range.